1. Field of the Invention
The present invention relates to a display device. In particular, the present invention relates to a display device having a touch-panel function (such display device is also referred to as a display device with a touch-panel function). Further, the present invention relates to an electronic device including the display device in a display portion.
2. Description of the Related Art
In recent years, mobile phones, portable music players, portable liquid crystal television, and the like have become popular with the development of communication technology. It is expected that a moving image will be transferred further and more amount of information will be transmitted in the future. On the other hand, a personal computer (PC) for mobile use has been produced due to the weight reduction. Information terminals called PDAs (personal digital assistant) such as electronic databooks are produced in large amount and coming into wide use. In addition, with development of display devices, the majority of such portable information devices are provided with a flat panel display.
Moreover, in recent years, in an active matrix display device, productization of a display device using a low-temperature polysilicon thin film transistor (hereinafter a thin film transistor is denoted to as a TFT) has been promoted. In the case where low-temperature polysilicon TFT is used, since a signal line driver circuit can be formed integrally in the periphery of a pixel portion as well as a pixel, a display device can be made smaller and have higher definition. Such a display device is expected to be more widely used in the future.
As electronic devices using such active matrix display devices, electronic devices having a touch-panel function are coming into wide use. The touch-panel function is to input information not by using a keyboard but by touching a display screen with a finger or a pen. Note that in the documents of this application (specification and claims), not being limited to inputting information by bringing an object to be read such as a finger or a pen into contact with a display screen, the touch-panel function includes inputting information with light, for example, inputting information with reflected light from an object to be read such as a finger or a pen. In the case where information is input with light, the illuminance of light which enters a reading portion is read as the information. As an example of the above, reference shown below can be given.